Résumé
In everyday life, cardiac automaticity is generated in the sino-atrial node (SAN) by specialised myocytes called pacemaker cells. Failure to generate a normal SAN impulse causes Sinus Node Dysfunction (SND). SND may be inherited (primary SND) or secondary to systemic or cardiovascular conditions (secondary SND). We have previously demonstrated that genetic ablation or pharmacologic inhibition of G protein gated K+ GIRK4 channels abrogates bradyarrhythmia in models of primary and secondary SND, but the mechanisms underlying rescuing of heart rhythm are not fully elucidated. In this context, a decrease in intrinsic electrical coupling due to progressive tissue fibrosis has been proposed as a primary factor in age-related SND, but its role in other forms of SND is currently unknown.
To test if genetic inhibition of Girk4 channel rescues cardiac conduction defects and fibrosis in Cav1.3-/- mouse model of primary SND.
We performed telemetric electrocardiogram recordings in control, Girk4-/-, Cav1.3-/- and GIRK4 -/-/Cav1.3-/- mice. Animals were then euthanized, and SAN tissues were isolated for investigations of pacemaker activity by optical and electrical mapping. Immunohistochemistry (IHC) staining was performed to study fibrosis in SAN tissues.
P-wave duration, PR and RR intervals were significantly longer in Cav1.3-/- than in WT and GIRK4 mice, suggesting conduction troubles and enhanced fibrotic tissue in Cav1.3-/- animals. Genetic ablation of Girk4 channels in Cav1.3-/- animals significantly shorten P-wave duration (in ms: 17.7±0.8 Cav1.3-/- vs. 14.3±0.2** GIRK4-/-/Cav1.3-/-) and both PR (in ms: 59±1 Cav1.3-/- vs. 44±1*** GIRK4-/-/Cav1.3-/-) and RR intervals (172±12 Cav1.3-/- vs. 137±5* GIRK4-/-/Cav1.3-/-) to WT values. Shift of the leading pacemaker site (defined by optical mapping experiments) and defects of conduction (quantified by fractionation index in MEA electrograms) recorded in Cav1.3-/- SAN isolated tissue were restored to control condition by genetic ablation of Girk4 in SND tissue. IHC experiments showed more 8% of fibrotic tissue (Sirius red histology) in Cav1.3-/- than in WT SAN tissue. GIRK4-/-/Cav1.3-/- SAN tissue fibrosis was strongly ameliorated.
In the context of SND generated by Cav1.3 ablation, concomitant inhibition of Girk4 normalises heart rate by restoring normal cardiac conduction pathway by limiting fibrosis in SAN tissue.